Hydrodynamics Natural Water Bodies Part 10 pdf

Therefore, the Yangtze River Estuary has North Branch, North Channel, North Passage and South Passage four outlets through which the water and sediment from the Yangtze River discharge i

Moreover, corresponding to the wave-frequency peak and low-frequency peak of the frequency spectra of the cable tensions, there occur the wave-frequency motions and low-frequency motions in the tunnel element This also reflects directly the interrelation of the tunnel element motions and the cable tensions

4 Conclusions

The motion dynamic characteristics of the tunnel element and the tensions acting on the controlling cables in the immersion of the tunnel element under irregular wave actions are experimentally investigated in this chapter The irregular waves are considered normal incident and the influences of the immersing depth of the tunnel element, the significant wave height and the peak frequency period of waves on the tunnel element motions and the cable tensions are analyzed Some conclusions are drawn as follows

As the immersing depth is comparatively small, the motion responses of the tunnel element are relatively large Besides the wave-frequency motions, the tunnel element has also the low-frequency motions that result from the actions of cables For the sway of the tunnel element, for different immersing depth the low-frequency motion is always larger than the wave-frequency motion While for the heave, with the increase of the immersing depth, the motion turns gradually from that the low-frequency motion is dominant into that the wave-frequency motion is dominant

For the large significant wave height, the motion responses of the tunnel element are accordingly large The peak values of the frequency spectra of the motion responses increase rapidly with the increase of the peak frequency period of waves Especially, for the heave motion of the tunnel element, the peak frequency of the response spectrum corresponding to the low-frequency motion increases with the increasing peak frequency period

The total force of the cables at the offshore side is larger than that of the cables at the onshore side of the tunnel element Corresponding to the motion responses of the tunnel element, the cable tensions are relatively large and their variations are more complicated in the case as the immersing depth is small and the significant wave height and the peak frequency period are large comparatively The changing laws of the tunnel element motions and the cable tensions reflect the interrelation of them

In this chapter, the immersion of the tunnel element is done from the fixed trestle in the experiment, by ignoring the movements of the barges on the water surface Actually, when the movements of the barges are relatively large, they have influences on the motions of the tunnel element The influences of the movements of the barges on the tunnel element motions will be considered in the further researches The numerical investigation will also

be carried out on the motion dynamics of the tunnel element in the immersion under irregular wave actions

5 Acknowledgment

This work was partly supported by the Scientific Research Foundation of Third Institute of Oceanography, SOA (Grant No 201003), and partly by the National Natural Science Foundation of China (Grant No 51009032)

6 References

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(2007) Nonlinear Response of Deep Immersed Tunnel to Strong Seismic Shaking,

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2007), pp 1067-1090, ISSN 1090-0241

Aono, T., Sumida, K., Fujiwara, R., Ukai, A., Yamamura K & Nakaya, Y (2003) Rapid

Stabilization of the Immersed Tunnel Element, Proceedings of the Coastal Structures

2003 Conference, pp 394-404, ISBN 978-0-7844-0733-2, Portland, Oregon, USA,

August 26-30, 2003

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7-03-010112-X, Beijing, China (in Chinese)

Chen, Z J., Wang, Y X., Wang, G Y & Hou, Y (2009a) Frequency responses of immersing

tunnel element under wave actions, Journal of Marine Science and Application, 2009,

Vol 8, pp 18-26, (March 2009), ISSN 1671-9433

Chen, Z J., Wang, Y X., Wang, G Y & Hou, Y (2009b) Time-domain responses of

immersing tunnel element under wave actions, Journal of Hydrodynamics, Ser B, Vol

21, No 6, (December 2009), pp 739-749, ISSN 1001-6058

Chen, Z J., Wang, Y X., Wang, G Y & Hou, Y (2009c) Experimental Investigation on

Immersion of Tunnel Element, 28th International Conference on Ocean, Offshore and Arctic Engineering, pp 1-8, ISBN 978-0-7918-4344-4, Honolulu, Hawaii, USA, May

31–June 5, 2009

Ding, J H., Jin, X L., Guo, Y Z & Li., G G (2006) Numerical Simulation for Large-scale

Seismic Response Analysis of Immersed Tunnel, Engineering Structures, Vol 28, No

10, (January 2006), pp 1367-1377, ISSN 0141-0296

Gursoy, A., Van Milligen, P C., Saveur, J & Grantz, W C (1993) Immersed and Floating

Tunnels, Tunnelling and Underground Space Technology, Vol.8, No.2, (December

1993), pp 119-139, ISSN 0886-7798

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(October 1996), pp 479-483, ISSN 0886-7798

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Construction Requirements of the Istanbul Strait Immersed Tunnel, Tunnelling and Underground Space Technology, Vol 20, (October 2005), pp 604-608, ISSN 0886-

7798

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Stability of an Immersed Tunnel in Offshore Conditions under Deep Water Wave

Impact Coastal Engineering, Vol 55, No 9, (August 2008), pp 753-760, ISSN

3783-3839

Zhan, D X & Wang, X Q (2001a) Experiments of hydrodynamics and stability of

immersed tube tunnel on transportation and immersing Journal of Hydrodynamics, Ser B, Vol 13, No 2, (June 2001), pp 121-126, ISSN 1001-6058

Zhan, D X., Zhang, L W., Zhao, C B., Wu, J P & Zhang, S X (2001b) Numerical

simulation and visualization of immersed tube tunnel maneuvering and

immersing, Journal of Wuhan University of Technology (Transportation Science

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(in Chinese)

Formation and Evolution of Wetland and Landform in the Yangtze River Estuary Over the Past 50 Years Based on Digitized Sea Maps and

Multi-Temporal Satellite Images

Fig 1 The sketch map of the Yangtze River Estuary

-60 -50 -40 -30 -20 -10 -5 0

Jiuduansha Shoal

1959 1979

2001 1990

Fig 3 Longitudinal section at 121°35′E, 31°16′N-122°25′E, 31°5′N (shown on Fig 2 as A-A')

of the Jiuduansha Shoal from 1959 to 2001

North Passage

North Channel 1953

1997

1965 1986

1959 1979

2001

1990

Eastern Hengsha Tidal Flat

Fig 4 Sketch map of the cross section at 122°E (shown on Fig 2 as B-B') of the Yangtze River Estuary from 1953 to 2001

2 Data and methodology

In order to analyze the formation and evolution of the Yangtze River Estuary in past 50

years, related sea maps from 1945 to 2001 and satellite images from 1975 to 2001 are

collected and analyzed Landsat MSS (multi-spectral scanner) data acquired on 1975 and

1979, Landsat TM (Thematic Mapper) and Landsat ETM+ (Enhanced Thematic Mapper

Plus) from 1990 to 2001, ASTER (Advanced Spaceborne Thermal Emission and Reflection

Radiometer) data from2002 to 2005 were collected and analysed All these remote sensing

data were corrected geometrically Image processing of these satellites remote sensing data

were used ENVI4.6 and Erdas9.0 And formation and evolution of the landform over the

past 50 years are analyzed in detail

3 Formation and evolution of wetland and landform

3.1 Formation mechanism of the Yangtze River Estuary

The Yangtze River Estuary is nearly 90 km wide at the mouth from the Southern cape to the

Northern cape Coriolis force and centrifugal force are strong enough to cause a horizontal

separation of the flow, forming an ebb tide dominated channel and a flood tide dominated

channel, respectively Because of the river bed friction, tidal currents and wave power

decreased during the tidal currents flow into the mouth and wave form began to change,

and the flood tidal range in the northern part is larger than that in the southern part of the

same cross section, while in the ebb tide period, the longitudinal water surface gradient and

the transverse water surface slope increase (Zhang and Wang, 1987) The transverse water

surface slope caused by curve bend circulation is J B,

2 cp

Vg

grC

where C is the Chézy roughness coefficient, V cp is the vertical mean velocity, r is the river

bend radius of curvature, and g is the acceleration of gravity For example, when V cp = 2

m/s，r = 10,000 m，C = 90 m1/2/s，and g = 9.81 m/s2, then JB = 4.1×10-5

Another factor that might affect transverse water surface slope in the Yangtze River Estuary

is the Coriolis force The transverse water surface slope caused by the Coriolis force was

studied by Zou (1990), in this case the transverse water surface slope is J C,

cp C

2 V sinJ

g

where  is the rotational angular velocity of the earth,  ＝7.27×10-5(s-1);  is the stream

section latitude,  is 32° If V cp is equal to 2.0 m/s in the calculation like in curve bend

circulation, and g is 9.81 m/s2, then J C = 1.57×10-5

Comparing J C and JB, shows that for similar condition, the slope caused by the Coriolis force

is smaller than that caused by curve bend circulation However, due to the long term action

of the Coriolis force, the thalweg of the ebb current and river flow is directed to the right

bank and formed the Ebb Channel, while the thalweg of the flood current is directed to the

left and formed the Flood Channel, the main tide direction is nearly 305° progressing from

the East China Sea toward the river mouth area while the ebb tide current direction is nearly

90°-115° The ebb tide current is not in a direction opposite to the flood tide direction; there

is a 10°-35° angle between the extension line of the flood and ebb tidal currents because of the Coriolis force(Shen et al., 1995) Ebb tidal current is obviously diverted to the south, while the flood current is diverted to the north Thus, between the flood and ebb tidal currents in the river mouth area there is a slack water region where sediment rapidly deposited to form shoals, and eventually coalesced to form estuarine islands (Chen et al., 1979) This is the evolutionary history of the three larger islands (Chongming Island, Changxing Island and Hengsha Island, respectively) in the estuary These islands form three orders of bifurcation and four outlets in the Yangtze River Estuary The first order of the bifurcation is the North Branch and the South Branch separated by Chongming Island The South Branch is further divided into the North Channel and the South Channel by Changxing Island and Hengsha Island The South Channel is further divided into the North Passage and the South Passage by the Jiuduansha Shoal (Fig 1) Therefore, the Yangtze River Estuary has North Branch, North Channel, North Passage and South Passage four outlets through which the water and sediment from the Yangtze River discharge into the East China Sea

From 1950 to 2003, the annual water discharge at the Datong Hydrologic Station did not substantially change The total annual discharge is about 9481×108 cubic meters per year and the sediment load is about 3.52×108 tons/yr The sediment discharge during the flood season (from May to October) constituted 87.2% of the annual sediment load before the 1990s, but decreased in the 1990s (Fig 5) Most of the suspended sediment are silt and clay, which are transported to the East China Sea where they are carried away from the delta by the longshore currents Part of the suspended load is deposited in mouth bars and a subaqueous delta area to form the tidal flats and mouth bars in the Yangtze Estuary A broad mouth bar system and tidal flats were formed The runoff and the sediment discharge

Annual water runoff Sediment load

Fig 5 Water and Sediment discharge from 1950 to 2003 at Datong Hydrologic Station

during the flood season vary between 71.7 and 87.2 % of the annual total value based on data from the Datong Hydrologic Station According to previous research (Gong and Yun, 2002; Niu et al., 2005), at discharges greater than 60,000 m3/s at the Datong Hydrologic Station, the estuarine riverbed has obviously changed due to erosion and deposition; when the flood water discharge greater than 70,000 m3/s, can form new branches on the river and cluster ditches because of the floodplain flows, these changes affect the estuary and new navigation channel development In 1954 (from June 18th to October 2nd), the average water discharge at the Datong Hydrologic Station was about 60,000 m3/s, and the highest discharge was about 92,600 m3/s Water discharge greater than 60,000 m3/s, increase the water surface gradient and the sediment carrying capacity in the estuary (Yang et al., 1999) Estuarine sedimentation and landform features have been observed and studied in various settings around the world, including the Thames Estuary, Cobequid Bay, and the Bay of Fundy (Dalrymple and Rhodes, 1995; Knight, 1980; Dalrymple et al., 1990), as well as Chesapeake Bay (Ludwick, 1974) and Moreton Bay (Harris et al., 1992) These studies found that tidal bars in all these estuarine settings are important sedimentary features Because estuaries are areas where freshwater and seawater mix, the systems react very sensitively to small changes in geomorphology of the estuary, and the results can reveal the changes of the estuarine environment

According to the evolution history of the Yangtze River Estuary (Wang et al., 1981; Li et al., 1983; Qin and Zhao, 1987; Qin et al., 1996; Chen et al., 1985, 1991; Chen and Stanley 1993, 1995; Stanley and Chen, 1993; Hori, K et al., 2001a, 2001b, 2002; Saito, Y et al., 2001), the main delta was formed by the step-like seaward migration of the river mouth bars from Zhenjiang and Yangzhou area, the apex of the delta, to the present river mouth (Fig 6) The newer generation island is Jiuduansha Shoal, it was once the southern part of the Tongsha Tidal Flat In 1945, under the processes of ebb and flood tidal currents, one pair of a

Fig 6 Evolution history of the Yangtze River Estuary (after Chen et al., 2000)

flood channel and an ebb channel developed on the southern part of the Tongsha Tidal Flat, but the Jiuduansha Shoal had not formed as an isolated shoal (Fig.7)

In 1954, the ebb channel and flood channel on the Tongsha Tidal Flat linked up, the linked ebb and flood channel formed the North Passage under the Flood from the drainage basin While the -2 m isobath line linked up the ebb channel and the flood channel, the Jiuduansha Shoal was isolated, and the Jiuduansha Shoal formed as a new island in the Yangtze River Estuary (Fig.8)

Fig 7 Former Jiuduansha Shoal in 1945

Passage

Lat.

-15 -10 -5 -2 0 2(m)

Fig 8 The Jiuduansha Shoal and the North and South Passage in 1959

The formation and landform evolution of the Yangtze River Estuary are related to the water and the sediment coming from the drainage basin and human activities, and also related to the riverine and marine processes The Yangtze River Estuary is an irregular semidiurnal tidal estuary, there is a clearly different tidal range in a day, especially, the daily mean higher high tide is 1.47 m higher than lower high tide (Shen and Pan, 1988) in a tidal cycle, a flow diversion period exists, and this period differs throughout the year because of the different flood and dry seasons, and different spring and neap cycles The channel bed changes easily and frequently under the actions of the runoff and the tidal current, while the human activities such as reclamation and navigation channel construction is also influence the landform features

3.2 Field survey evaluation

In order to study the relation between the deposition and erosion of the tidal flat during the flood and dry season at spring and neap tides, field survey data for the middle section of the North Passage and the South Passage are analyzed The velocity and sediment concentration

in the North Passage and the South Passage during the spring tidal cycle obtained in the field survey use OBS 5 and DCDP and water and sediment samples which measured in the laboratory, part of the related results are shown in Fig 9 and Fig 10, and a summary of the collected data is listed in Table 1

Data from this field survey show that the flow velocity and sediment concentration in the dry and flood seasons at spring and neap tidal cycles are different In the dry season during spring tide in the South Passage, the flow velocity at the water surface (H is the relative water depth, the surface is 0H, 1H is the bottom) in the ebb tide period is higher than that in the flood tide period (Table 1) At a relative depth of 0.4H, the ebb tide velocity is lower than that the flood tide current At 0.8H relative depth from the water surface, the flow velocity

of the ebb tide is lower than that the flood tide current In the neap tidal cycle in the South Passage, the ebb tide and river flow velocity at relative depth of 0H and 0.4H depth are higher than that flood tide velocity respectively, but the flood velocity at relative depth of 0.8H is higher than that ebb and river flow velocity

In the dry season during spring tide in the North Passage, the velocity of ebb tide and river flow at relative depth of 0H is little lower than that flood velocity, but at relative depth of 0.4H and 0.8H are little higher than that flood velocity, respectively While during the neap tide period, the ebb and river flow velocity at relative depth of 0H, 0.4H, and 0.8H are higher than that flood tide velocity respectively

In the flood season during the spring and neap tidal cycle in the South and North Passage, the ebb tide and river flow velocity at relative depth of 0H, 0.4H, and 0.8H depth are correspondingly higher than that flood velocity, respectively

In most cases, the mean sediment concentration during ebb tide period in the South and North Passage in the dry and flood season during the spring tidal cycle at relative depth of 0H, 0.4H, and 0.8H are higher than that flood tide period, respectively But in some cases, the sediment concentration at relative depth of 0H and 0.4H are different because of the different riverine mechanics during the spring and neap tidal cycle

Through the comparison of the velocity of ebb tide and river flow with flood tide velocity during the spring and neap tidal cycle in flood and dry season, in most cases, the ebb tide and river flow velocity at water surface is higher than the flood tide velocity, while at relative depth of 0.4H and 0.8H, in some cases, the flood tide velocity is higher than that the ebb tide and river flow velocity That is during the flood tide period, flood tide current start